Marine electromagnetic exploration is an important tool for locating off-shore hydrocarbon reserves and monitoring hydrocarbon production for reservoir management. One known procedure for marine electromagnetic involves the use of an electromagnetic source and receiver cables as described in the present applicants' WO 01/57555. Electromagnetic energy generated by the source propagates both upwards into the water column and downwards through the earth. The downward propagating waves are partially reflected and refracted by the subsurface layers. The reflected and refracted energy travels upwardly from the subsurface layers and is detected by the receiver array. In particular, hydrocarbon filled reservoirs are known to give strongly refracted energy which is of interest for hydrocarbon imaging.
Electromagnetic exploration however is complicated by waves received at the receiver array as downward-traveling reflections and refractions after reflecting and refracting off the air/water boundary at the surface. The air/water boundary is an efficient reflector and refractor, and thus the waves traveling downwards are difficult to differentiate from the upgoing waves from the subsurface. The downward-traveling energy is caused both by energy propagating directly from the electromagnetic source to the air/water boundary and by energy from the subsurface traveling to the air/water boundary.
Reflections and refractions from the sea surface thus are a severe problem. If the sea surface reflections and refractions are not properly attenuated, they may interfere and overlap with primary reflections and refractions from the subsurface. It is an object of the present invention to provide a method of processing an EM wavefield which minimizes this difficulty.
According to the invention, a method of processing an electromagnetic (EM) wavefield comprises resolving (or decomposing) the wavefield into upgoing and downgoing compounds and then analyzing the upgoing component. Optimal processing, analysis and interpretation of electromagnetic data ideally require full information about the wavefield so that the wavefield can be resolved into its upgoing and downgoing constituents.
At a position just above or below the seabed the sea surface reflections and refractions are always downgoing wavemodes. The reflections and refractions of interest from the subsurface, however, are upgoing wavemodes. Resolution (or decomposition) of the electromagnetic wavefield into upgoing and downgoing constituents just above or below the seabed puts the sea surface reflections and refractions into the downgoing component whereas the subsurface reflections and refractions are contained in the upgoing component.
Thus, it is a further object of the present invention to provide a technique that resolves (or decomposes) the electromagnetic wavefield recorded along one or several receiver arrays into upgoing and downgoing wave components.